1. Field of the Invention
The present invention relates to a transfer molding machine for encapsulation of semiconductor devices with a resin compound and, more particularly, to an electrically-driven transfer molding machine for encapsulation of semiconductor devices with a thermosetting resin without causing resin flash on a lead frame having semiconductor devices bonded thereto.
2. Description of the Prior Art
A transfer molding machine used for encapsulation of semiconductor devices with a thermosetting resin, generally comprises a clamping device, a transfer mold arranged therein, a resin transfer mechanism for injecting a resin compound from a pot into cavities of the mold, and an ejector mechanism for ejecting the molded products from the cavities. The clamping device includes a stationary base platen; four tie rods fixed at one end thereof to the base platen; a stationary upper platen mounted on the opposite ends of the tie rods, to which an upper mold section of the transfer mold is directly bolted; and a moving lower platen movably mounted on the tie rods, to which a lower mold section of the transfer mold is directly bolted. The clamping device is generally provided with a hydraulic cylinder connected directly or through a toggle mechanism to the moving platen to exert an opening and closing force and hold the mold closed during a curing time.
In operation, a thermosetting resin compound is loaded into a pot of the mold and then one or more lead frames each having semiconductor devices or chips previously bonded thereto are loaded into cavities of the lower mold section. The moving lower platen is actuated by the hydraulic clamping mechanism to close the mold. Then, the thermosetting resin compound in the pot is transferred to the cavities of the closed mold through gates whereby the semiconductor devices are enclosed in the resin. After curing, the mold is opened and the molded pieces are ejected from the cavities of the mold by the ejector mechanism.
However, the hydraulic clamping mechanism has a serious disadvantage that an actuating oil often leaks from the hydraulic cylinder and exerts bad influences on the quality of the semiconductor devices. Thus, there is an increasing demand for development of a clean transfer molding machine for encapsulation of semiconductor devices, free from the leakage of the actuating oil.
To meet this demand, inventors have tried to use an electric motor to exert the opening and closing force and hold the mold closed during curing time. In this case, it is required to reduce a load of an electric motor during a cooling time for final curing of the thermosetting resin. This can be done by a combination of the electric motor with a toggle mechanism, i.e., by an electrically-actuated toggle mechanism.
However, it has now been found that the clamping device with the electrically-actuated toggle mechanism causes resin flash on the lead frame. As is well-known, the toggle mechanism is limited in the maximum stroke for applying the opening and closing force to the moving platen by the size of its links. In case of the hydraulic clamping mechanism, the hydraulic cylinder leaves a sufficient margin for upper stroke of the links whereby the difference in precision of the links is compensated completely.
Different from the hydraulic clamping mechanism, the electrically-actuated toggle mechanism has no margin for the upper stroke of the links, thus making it impossible to compensate for the difference in precision of the links. For this reason, it is unable to hold the moving platen of the clamping mechanism in parallel to the upper stationary platen when the mold is closed. Both the upper and lower mold sections are directly bolted to the stationary top platen and moving platen, thus making it impossible to hold mating surfaces of the mold sections parallel with one another. This provides a slight gap between the mold sections of the closed mold and the lead frame loaded therein so that the resin can flash on the lead frames and exerts bad influences on the appearance and quality of the products. In addition, the flash must be removed from the lead frame before the products are put on the market, thus making it difficult to make all the transfer molding operations automatic.
Further, even with careful design of the mold, resin flash occurs on the lead frame when the mold becomes worn or when close tolerance on lead frames is not maintained. In the hydraulic-actuated transfer molding machine, some techniques have been proposed, for example, in Japanese patent JP-B- 61-3648 and Japanese Utility model JP-B- 3-6410 to prevent the resin flash.
Japanese patent JP-B- 61-3648 discloses a transfer molding machine including a transfer mold with floating cavity blocks. The use of the floating cavity block mold fulfills its function at the initial stage of usage, but its function would be lost gradually as the resin would flash in gaps formed between floating cavity blocks and mold chases or retainer plates.
Japanese Utility model JP-B- 3-6410 discloses a transfer molding machine including a mold with compressible cavity blocks. However, the compressible cavity blocks make the mold complicated, resulting in considerably increase in manufacturing cost of the transfer mold.